A study in obese men with a low to low-normal testosterone - typical for the majority of obese men - has reported that while dieting men receiving placebo lost both fat and lean mass, the weight loss with testosterone treatment was almost exclusively due to loss of body fat.

The study, ‘Effects of testosterone treatment on body fat and lean mass in obese men on a hypocaloric diet: a randomised controlled trial’, published in BMC Medicine, by researchers from the University of Melbourne and Monash University, Melbourne, Victoria, Australia, hypothesised that following diet-induced loss of fat mass, testosterone treatment will prevent fat regain but maintain lean mass.

They conducted a randomised double-blind, parallel, placebo controlled trial at a tertiary referral centre and recruited 100 obese men (BMI≥ 30) with a total testosterone level of or below 12nmol/L and a median age of 53 years (range 47–60) receiving ten weeks of a very low energy diet (VLED) followed by 46 weeks of weight maintenance were randomly assigned at baseline to 56 weeks of ten-weekly intramuscular testosterone undecanoate (n=49, cases) or matching placebo (n=51, controls). The main outcome measures were the between-group difference in fat and lean mass by dual-energy X-ray absorptiometry, and visceral fat area (computed tomography).

Outcomes

A total of 82 men completed the study and compared to controls, cases had greater reductions in fat mass, with a mean adjusted between-group difference (MAD) of 2.9kg (–5.7 to –0.2; p=0.04), and in visceral fat (MAD –2678 mm2; –5180 to –176; p=0.04). Although both groups lost the same lean mass following VLED (cases –3.9 kg (–5.3 to –2.6); controls –4.8kg (–6.2 to –3.5), p=0.36), cases regained lean mass (3.3 kg (1.9 to 4.7), p<0.001) during weight maintenance, in contrast to controls (0.8 kg (–0.7 to 2.3), p=0.29) so that, at study end, cases had an attenuated reduction in lean mass compared to controls (MAD 3.4 kg (1.3 to 5.5), p=0.002).

As the combined lean and fat mass lost in controls was similar to the amount of fat mass lost in the cases, the difference in body weight change at study end was no different between groups (MAD –0.5 kg (–4.3 to 3.3), p=0.80). Age, baseline total testosterone, luteinising hormone and SHBG levels were all not predictive of changes in body composition after 56 weeks in the trial. In addition, baseline fat mass did not interact with the changes in body composition. In addition, adjustment for physical activity did not alter the findings.

In an additional sensitivity analysis, when re-analysed with a robust linear mixed model, the main outcome fat loss after 56 weeks was more pronounced in the testosterone group, compared to the placebo group (–4.1 kg (–5.6 to –2.7), p<0.01).

“The major novel findings of this RCT are that, among obese men with low to low-normal testosterone submitted to a weight loss program, testosterone treatment decreased total fat mass and visceral adipose tissue, and protected against loss of total and appendicular lean mass,” the authors report.

However, differences did emerged in the weight maintenance phase, during which men receiving testosterone maintained weight loss (p=0.62), while there was marginal weight regain in the placebo group (p=0.06). At the end of the study, there were marked differences in body composition between groups, and men receiving testosterone had greater reductions of fat mass (–2.9 kg) and visceral fat area (–2678mm2) compared to placebo.

They added that the results indicate that, compared to men receiving placebo who lose both fat and muscle mass during diet, testosterone treatment shifts this weight loss to almost exclusive fat mass loss.

“Among obese men with a low to low-normal testosterone typical for the majority of obese men, testosterone treatment augmented diet-induced loss of total fat and visceral fat mass, and preserved lean mass so that, in contrast to placebo-treated men who lost both lean and fat mass, diet-induced weight loss during testosterone treatment was almost exclusively due to the loss of body fat,” the researchers concluded.